Radiotelephone communications is one of the 20 fastest growing services currently offered by the telecommunications industry. The underlying technology which supports such services is well documented and well within the knowledge and understanding of those skilled in the art. For a more detailed discussion, the interested reader may refer to the following publications, each of which is incorporated herein by reference:
EIA/TIA Interim Standard IS-41-A: "Cellular Radiotelecommunications Intersystem Operations" January 1991;
EIA/TIA Interim Standard IS-54: "Cellular System Dual-Mode Mobile Station--Base Station Compatibility Standard" May 1990;
EIA/TIA Standard EIA/TIA-553: "Mobile Station--Land Station Compatibility Standard" September 1989; and
U.S. Pat. No. 4,901,340-Parker Et. Al. "System for the Extended Provision of Cellular Mobile Radiotelephone Service."
One of the attractive features of radiotelephone (RT) service is that it affords the RT subscriber more convenient access to and from the public switched telephone network (PSTN). Consequently, RT subscribers are accustomed to placing and receiving telephone calls, without the need for, or availability of PSTN telephones. The advent of portable RT units has only operated to further increase RT subscriber accessibility.
Unfortunately, certain features of currently existing radiotelephone communication systems are at direct odds with this trend towards greater accessibility. Heretofore, an RT subscriber's ability to receive incoming calls has been severely restricted when the subscriber is not physically present within a specified geographic area. For example, calls placed to a subscriber's phone number will frequently be answered by a message indicating that the subscriber cannot be found. This is often the case when the subscriber is "roaming". By definition, a subscriber using or attempting to use his radiotelephone in a service area different from his designated "home" area is said to be "roaming" and may therefore be considered a "roamer".
In a typical configuration of multiple, non-networked RT systems, in order for a roaming RT subscriber to receive incoming calls, would-be callers need to know in which geographic area the subscriber is physically present and the procedures for accessing the subscriber through the facilities of that geographic area's servicing system. Such systems typically have a ten digit roaming access number. Areas in which service is provided by both wire line and non-wire line carriers have at least two such access numbers, corresponding to the wire line and the non-wire line carrier, respectively.
Armed with the appropriate access number, the would-be caller must first dial the ten digit access number of the system serving the area he expects the subscriber to be within, followed by the RT subscriber's phone number. This offtimes awkward method can be further complicated by the dialing of incorrect access numbers, the subscriber having to prearrange delivery of incoming calls, subscriber absenteeisms, not to mention the steps required to satisfy various billing arrangements.
Since roaming is a condition unique to, yet frequently experienced by the average RT subscriber, demand dictates that improved service be offered this expanding sector of the RT subscriber population. It would be extremely advantageous therefore to provide a method for continuous RT subscriber accessibility, regardless of geographic location within a servicing RTN.
This problem has been the subject of substantial activity by committees of industry experts and by private companies over a number of years culminating in the specification of various schemes for "knitting together" multiple individual RT systems into a complete RT Network (RTN) within which RT subscribers may receive their normal services automatically regardless of their actual physical location within the RTN. For example, the
reference EIA/TIA Interim Standard IS-41-A: "Cellular Radiotelecommunications Intersystem Operations" provides an example of a public standard, while U.S. Pat. No. 4,901,340-Parker et. al. offers a proprietary solution for networked RT systems.
A major issue in the overall RTN solution has been the problem of how to establish a connection through the PSTN between the RTN element where an incoming call to a RT subscriber first appears in the RTN and the element then controlling the radio service area in which the RT subscriber is operating. Both the above cited references specify a technique employing a "Temporary Directory Number" (TDN) assigned by the current serving RTN element to establish the desired connection. The present invention takes a different approach to establishing the same connection.